Recent evidence indicates that acyl transfer reactions of activated ester compounds occur by a concerted process, not via a tetrahedral intermediate as has been generally assumed. Isotope effect studies in this lab have confirmed this and have given a picture of the structure of the transition states for some of these reactions. Such activated esters have long been used in the study of hydrolytic enzymes, which have evolved to stabilize tetrahedral intermediates. The behavior of these activated substrates when they are hydrolyzed by enzymes will answer questions about the relevance of such substrates in the study of these enzymes, and shed light on the effects of catalytic groups on the mechanistic course of a reaction. Proton NMR is routinely used to characterize compounds in the synthetic work involved in the project.